This application is based on Japanese patent application No. 9-330414 filed on Dec. 1, 1997, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to image signal processing technologies, and more particularly to technologies of processing image signals picked up with a solid state image pickup device.
b) Description of the Related Art
FIGS. 2A and 2B show a solid state image pickup device according to conventional technologies. For example, the image pickup device can pick up an image of high resolution of 1530xc3x971024 pixels. FIG. 2A illustrates a control method for the solid state image pickup device operating in a first mode, and FIG. 2B illustrates a control method for the solid state image pickup device operating in a second mode.
Referring to FIG. 2A, the first mode is an all-pixel read mode and reads all pixels, for example, 1530xc3x971024 pixels, of the image pickup device. The first mode is used, for example, for printing an image of high precision with a printer.
The solid state image pickup device has photodiodes PD, vertical transfer paths 1, a horizontal transfer path 2, and an amplifier 3. The photodiodes PD as many as the number of pixels forming a full image are actually disposed in a two-dimensional matrix shape. For example, the number of pixels in the horizontal direction is 1530, and the number of pixels in the vertical direction is 1024. In FIG. 2A, however, one column is shown constituted of nine photodiodes PD1 to PD9, for the simplicity of drawing. All or each of the photodiodes PD1 to PD9 is called hereinafter a photodiode PD where applicable.
The solid state image pickup device has a plurality of photodiodes PD disposed in a two-dimensional shape and a plurality column of vertical transfer paths 1. One photodiode corresponds to one pixel constituting a two-dimensional image and converts received light into electric charges.
In the first mode, charges are read from all the photodiodes PD1 to PD9 and supplied to the right vertical transfer paths 1. The vertical transfer path 1 has four electrodes per one photodiode PD. The four electrodes are supplied with transfer pulses V1 to V4. The vertical transfer path 1 is four-phase driven with the transfer pulses V1 to V4 to transfer charges in the vertical direction.
The charges on the vertical transfer path 1 are transferred in the downward vertical direction to the horizontal transfer path 2. The horizontal transfer path 2 transfers the charges in the leftward horizontal direction. The amplifier 3 amplifies the charges transferred from the horizontal transfer path 2 and outputs amplified image signals.
Referring to FIG. 2B, the second mode is a thinning read mode. For example, an image of 1530 (H) and 1024 (V) pixels is thinned to read an image of 1530 (H)xc3x97256 (V) pixels. Namely, three pixels among four pixels are thinned in the vertical direction to read 256 pixels among 1024 pixels.
For example, the second mode is used for displaying an image on a small liquid crystal display mounted on a camera for adjusting an angle of view or is used for reading an image when auto focus is performed. The second mode is required to read one-field image at high speed ({fraction (1/60)} sec to {fraction (1/30)} sec).
In the second mode, electric charges are read from every fourth pixels, photodiodes PD1, PD5, and PD9 in the vertical direction, and supplied to the right side vertical transfer paths 1. Sets of four electrodes of the vertical transfer path 1 are supplied with the transfer pulses V1 to V4 similar to the first mode. The vertical transfer path 1 is four-phase driven with the transfer pulses V1 to V4 to transfer charges read from every fourth pixels along the vertical direction, in the downward vertical direction. The horizontal transfer path 2 transfers charges received from the vertical transfer paths 1 in the leftward horizontal direction. The amplifier 3 amplifies the charges transferred from the horizontal transfer path 1 and outputs amplified image signals.
The solid state image pickup device has such first and second modes. In any of the first and second modes, the vertical transfer path 1 is four-phase driven with the four electrodes. Next, a four-electrode four-phase drive method will be described.
FIG. 3 is a timing chart of the transfer pulses V1 to V4. The abscissa represents a time t, a unit time being an overlap time of one transfer pulse upon another transfer pulse.
FIG. 4 is a potential transition diagram of the vertical transfer path where the ordinate represents the time t shown in FIG. 3 and the abscissa represents a vertical position along the vertical transfer path 1. For example, eight photodiodes PD1 to PD8 are disposed in the vertical direction and connected to one vertical transfer path. Four electrodes per one photodiode PD are provided on the vertical transfer path. The four electrodes are supplied with the transfer pulses V1 to V4. Electric charges are accumulated in a low potential region. As seen from this potential transition diagram, as the potential changes, electric charges are transferred from the right to left in FIG. 4 along the vertical transfer path. Hatched photodiodes PD1 and PD5 are read in the thinning second mode.
Hatched charges 5 are at the height of the vertical transfer path same as that of the photodiode PD5 at the time t=0. As the time lapses, the charges 5 are transferred in the vertical direction (left side in FIG. 4). At time t=32, the charges are at the height of the vertical transfer path same as that of the photodiode PD1. It takes therefore 32 cycles to transfer the charges 5 from the height of the photodiode PD5 to the height of the photodiode PD1.
As shown in FIG. 4, one charge accumulation region (packet) per one photodiode PD is formed along the vertical transfer path. The packet is a region where electric charges can be accumulated. There are a packet with charges and a packet (empty packet) without charges. It is possible to efficiently transfer electric charges along the vertical transfer path in the first mode.
However, in the second mode (thinning read mode), for example, three packets among four packets are thinned. Therefore, the three packets among the four packets along the vertical transfer path are wasteful packets. It cannot be said therefore that the charge transfer along the vertical transfer path is efficient in the second mode.
It is an object of the present invention to provide a solid state image pickup device and an electric charge transfer method capable of transferring electric charges in the vertical direction at high speed in both an all-pixel read mode and a thinning read mode.
According to one aspect of the present invention, there is provided a solid state image pickup device comprising: mode selecting means for selecting one of first and second modes; a plurality of photoelectric converters for converting received light into electric charges; transfer means having a plurality of packets for receiving the electric charges from the plurality of photoelectric converters and transferring the electric charges in each packet; gate means for reading the electric charges from each of the plurality of photoelectric converters and supplying the read electric charges to the transfer means; and drive means for driving the transfer means in the selected first or second mode at the number of drive phases different from the number of drive phases of the non-selected mode.
For example, the first mode is an all-pixel read mode and the second mode is a thinning read mode. For example, the drive means drives the transfer means with four phases in the first mode and with sixteen phases in the second mode. The solid state image pickup device can drive the transfer means with the number of drive phases suitable for the selected mode.
The solid state image pickup device can transfer charges efficiently and at high speed both in the first and second modes. As the number of drive phases is increased, the charge transfer capacity can be increased.
According to another aspect of the present invention, there is provided a charge transfer method comprising the steps of: (a) selecting one of first and second modes; (b) reading electric charges from each of a plurality of photoelectric converters and supplying the read electric charges to transfer means; and (c) driving the transfer means in the selected first or second mode at the number of drive phases different from the number of drive phases of the non-selected mode.